Function validator

tools/ngui/static/js/lib/angular.js:12281–12289

(value)

      attr.required = true; // force truthy in case we are on non input element

      var validator = function(value) {
        if (attr.required && (isEmpty(value) || value === false)) {
          ctrl.$setValidity('required', false);
          return;
        } else {
          ctrl.$setValidity('required', true);
          return value;
        }
      };

      ctrl.$formatters.push(validator);
      ctrl.$parsers.unshift(validator);